Torrefaction is a thermal chemical treatment of biomass at 200° C. to 320° C. under atmospheric conditions and in the absence of oxygen, as opposed to pyrolysis where high heat is applied to the biomass at approximately 900° C. Torrefaction of biomass into a torrefied biomass (also called “biocoal”) can occur at a lower temperature if some volatiles are tolerated together with a chemical rearrangement of the C, H, and O atoms in solids. The biocoal generated from torrefaction is usable in existing coal-fired power plants. Various forms of biomass feedstock may be torrefied to form the biocoal. During torrefaction, water vapor is released from biomass, and continuous removal of the water vapor from the biomass helps to drive the desired decomposition of the green biomass.
The conventional torrefaction process has its shortcomings. Quality control is required to ensure a desired grindability of the biocoal as the power plants use pulverized coal with particle size less than 0.2 mm. Moreover, the biocoal should preferably have very low water content although moisture starts typically 50% by mass when the biomass is green. Further, during torrefaction, some of the volatiles generated during the torrefaction process must be driven off without losing too much of the potential fuel content.
All torrefaction processes require significant amounts of energy beyond what is required to convert biomass into biocoal. For example, it takes energy to chip the wood (or bamboo), briquette the corn stover (or switchgrass), heat the biomass to drive out the water from the biomass, transport the biomass to the torrefaction system, and transport the biocoal from the torrefaction system to a destination. Thus, it is important to achieve high energy efficiency at the production facility if the goal is to produce price-competitive biocoal. Since energy losses increase as the surface area increases, but production rate increases as the volume increases, high throughput is the key to achieving energy efficiency and economic competitiveness in the torrefaction process.
To reduce energy consumption, it has been proposed to use volatile organic compounds (VOCs) released from the biomass during torrefaction as a heat source. The VOCs may be sent to a reactor or furnace, which in turn generates flue gas to heat the biomass. This method has technical and economical problems. First, the flue gas is not an efficient heat transfer agent. Second, building a continuous process apparatus is more of a challenge with gas-based systems that must exclude air. With flue-gas batch processing, it takes many hours to get temperatures high enough to drive out water and heat the biomass to torrefaction conditions. Third, if the heat of torrefaction derives from burning a portion of the biomass, considerable smoke and soot is generated that contributes to air pollution. Fourth, when woody materials are used as the biomass, the woody materials do not generate sufficient VOCs during torrefaction. To obtain a higher quality product, the VOCs need to be mixed with natural gas, thereby increasing production costs and the carbon dioxide footprint.
It has also been proposed to use high temperature steam as a clean source to heat the biomass. However, water may permeate the final products, adversely affecting the quality of the biocoal.
U.S. Pat. No. 7,942,942 (“the '942 patent”) discloses an apparatus using a serpentine path of piping to torrefy biomass with hot paraffin or oil. In the '942 patent, biomass enters into an oxygen excluded environment via a pre-heating section of warm oil/paraffin, is torrefied in a middle section of hot oil/paraffin, and exits as biocoal via a post-cooling section of warm oil/paraffin. Each section is kept at a fixed temperature by external heating or cooling, resulting in additional energy loss, especially when water in the biomass is evaporated without the latent heat being recaptured in subsequent recondensation. Moreover, oil/paraffin remains trapped in the pores of the biocoal, and the residual oil/paraffin may constitute as much as 40% of the weight of the biocoal, making the biocoal unsuitable for pulverization for use in modern coal-fired power plants. Further, because oil costs more than the coal per unit heating value, the produced biocoal with large amounts of residual oil is not economically viable. A design where hot biocoal exits into a post-cooling oil port maintained at 280° F. (138° C.) and exposed to air also poses a safety issue. Residual VOCs like methanol and methane with low flash points released from the still hot biocoal or that migrate from the middle torrefying section to the end cooling section of a connected serpentine path could catch on fire or even cause an explosion.